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  1. Join Date
    Jul 2006
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    #31
    Quote Originally Posted by Alpha_One View Post
    Nowhere in the question did it say that the wind moves in the opposite direction. Assuming "no wind" (wind ang '0mph' reference) plane moves at 1MPH against the wind, runway moves at 1MPH against the wind in the opposite direction, plane and runway moves against each other at 2MPH.
    The only way to generate lift is if the wind is going opposite the plane's direction. You just proved my point. The fact is, there is NO wind. Hence, there is NO lift.

  2. Join Date
    Oct 2002
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    #32
    again, with the conveyor under the wheels, the plane is in a stationary position. you cannot create enough lift without the right wind velocity. the thrust created by the jet engines will only be negated by the conveyor runway.

  3. Join Date
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    #33
    Quote Originally Posted by boybi View Post
    again, with the conveyor under the wheels, the plane is in a stationary position. you cannot create enough lift without the right wind velocity. the thrust created by the jet engines will only be negated by the conveyor runway.
    Exactly my point!

  4. Join Date
    Aug 2004
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    #34
    The plane does NOT have to exert force on the runway to take off!

    That's the trick in this question.

    The plane only has to exert motive force on itself to push itself forward. Like Alpha One said, the wheels are only there to keep the plane off the ground. Once the plane has accelerated relative to the ground (not the conveyor), it will lift off.

    The only time the conveyor would keep the plane from taking off is if it matched the force exerted by the engines, and even then, you would have to have that amount of force exerted on the wheel hubs, not the wheels themselves!

    Thus, say a 747 has 80,000 pounds of thrust (I have NO idea what it does make, this is just an example), you would have to exert 80,000 pounds of force against the wheel hubs to keep the plane stationary.

    If the wheels of the plane are well lubricated and the tread is fresh, say the rolling resistance puts the drag transmitted from the conveyor at 5% of the total force exerted by the conveyor.

    Thus, you'd need, possibly, 1,600,000 pounds of force exerted by the conveyor on the wheels of the plane to keep it from taking off.

    This has nothing to do with the speed of the conveyor, just the torque exerted in moving it. And the torque/force/whatever of the conveyor is not specified, merely the speed.

    And if it matches the speed of the plane, it can match the speed with less force. It would actually have to be moving (given that theoretical 5% transmission that I conjured from thin air) at least twenty times faster to keep the plane from moving. If it is going ONLY at the same speed as the plane, the plane will take off.

    But, in reality (I mean, our virtual reality)... it's still difficult for the conveyor, as once the plane starts gaining momentum, the drag upon the wheels play less and less of an effect... and you would have to increase the force transmitted by the conveyor by making it run even faster.

    You would eventually reach the point where the conveyor is running at an infinite speed and transmitting almost no force to the wheels, as less and less of the weight of the plane is on the wheels.
    Last edited by niky; November 9th, 2006 at 07:21 PM.

    Ang pagbalik ng comeback...

  5. Join Date
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    #35
    Quote Originally Posted by mbeige View Post
    The plane's position, relative to the conveyor belt, remains the same because the force that the plane exerts on the conveyor belt is cancelled by the conveyor belt applying the same force but opposite in direction to the plane via the wheels. Therefore, the wheels rotate and act as a bearing, allowing the plane to remain on top of the conveyor belt. Like it is stated on the question, once the plane gains a certain speed, it is instantly brought to zero by the conveyor belt because it is already stated that the belt will instantly provide the same speed but opposite in direction. Hence, the plane's speed relative to the conveyor belt is zero.



    The runway can only exert the force to the wheels, not to the plane itself, because the wheels will be freewheeling.



    What do you mean?



    It cannot take off because it is stationary! Relative to the wind, the plane is stationary. Buti sana kung totoong runway yon, then yes. That's because the runway in this situation is moving on the opposite direction.
    The plane's position, relative to the runway, does change. In fact, their speed relative to each other would be double (not zero) that of a stationary runway.

    Paragraph 2: You just proved my point. The wheels are indeed freewheeling, they have absolutely nothing to do with the speed of the airplane! The runway exerts absolutely no force on the airplane itself (other than the wheels)!

    Paragraph 3: What I mean is, the speed of the runway has absolutely nothing to do with the speed of the airplane itself. The wheels will be spinning TWICE as fast, but the plane goes forward as usual.

    Paragraph 4: In fact, from a completely logical/philosophical perspective, such a scenario (the plane being stationary) is impossible unless the runway is stationary too. Premise: for the runway to do xMPH, the plane must do xMPH in the other direction. From a physics standpoint, it's also completely possible, the wheels would simply spin twice as fast. Other than that everything else is exactly the same as a perfectly normal, stationary runway.
    Last edited by Alpha_One; November 9th, 2006 at 07:25 PM.

  6. Join Date
    May 2005
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    #36
    Quote Originally Posted by boybi View Post
    it won't take off. kelangan ng wind drag to lift the airplane, walang kwenta kung mabilis ang gulong pero wala naman hangin.
    +1
    An airplane flies because its wings create lift, the upward force on the plane, as they interact with the flow of air around them. The wings alter the direction of the flow of air as it passes. The exact shape of the surface of a wing is critical to its ability to generate lift. The speed of the airflow and the angle at which the wing meets the oncoming airstream also contribute to the amount of lift generated.

  7. Join Date
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    #37
    Quote Originally Posted by BoEinG_747 View Post
    +1
    An airplane flies because its wings create lift, the upward force on the plane, as they interact with the flow of air around them. The wings alter the direction of the flow of air as it passes. The exact shape of the surface of a wing is critical to its ability to generate lift. The speed of the airflow and the angle at which the wing meets the oncoming airstream also contribute to the amount of lift generated.
    And here's where the trick to the problem comes in. Take note that in the problem it's the runway that's moving the opposite direction, not the wind.

    Treadmill guys, hindi wind tunnel.

  8. Join Date
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    #38
    Quote Originally Posted by Alpha_One View Post
    The plane's position, relative to the runway, does change. In fact, their speed relative to each other would be double (not zero) that of a stationary runway.

    Paragraph 2: You just proved my point. The wheels are indeed freewheeling, they have absolutely nothing to do with the speed of the airplane! The runway exerts absolutely no force on the airplane itself (other than the wheels)!

    Paragraph 3: What I mean is, the speed of the runway has absolutely nothing to do with the speed of the airplane itself. The wheels will be spinning TWICE as fast, but the plane goes forward as usual.
    Tell me, then, what happens if neither body exerts a force on the other? NOTHING. It does NOT move! Neither one moves the other!

    The plane's position does NOT change because it does not move!

    You're right, the speed of the runway has nothing to do with the plane speed. So they are NOT interacting, proving my first statement above. Hence, no forces acting upon each other. No movement. No movement, no lift.

  9. Join Date
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    #39
    Quote Originally Posted by mbeige View Post
    Tell me, then, what happens if neither body exerts a force on the other? NOTHING. It does NOT move! Neither one moves the other!

    The plane's position does NOT change because it does not move!

    You're right, the speed of the runway has nothing to do with the plane speed. So they are NOT interacting, proving my first statement above. Hence, no forces acting upon each other. No movement. No movement, no lift.
    Ah, of course the plane's position does change! The engines keep pushing it against the wind. The runway is doing absolutely no opposing force. The engine works on the plane, the treadmill works on the wheels. End result, plane moves, eventually it reaches takeoff speed.

    Indeed, the plane doesn't interact with the runway. But that's exactly why it's moving. It's the engines that interact with the plane. Now with the assumptions that the wheels have no bearing friction and all that rubbish, the wheels effectively isolate the runway-wheel interaction with the plane-engine interaction. Therefore, the engine is perfectly free to do what it wants to, which is propel the plane forward.

    Your statement isn't "proof" because it assumes that the engines are doing absolutely nothing to the plane.
    Last edited by Alpha_One; November 9th, 2006 at 07:33 PM.

  10. Join Date
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    #40
    Quote Originally Posted by Alpha_One View Post
    And here's where the trick to the problem comes in. Take note that in the problem it's the runway that's moving the opposite direction, not the wind.

    Treadmill guys, hindi wind tunnel.
    Kaya nga, there is NO wind going opposite in direction. Don't you realize that in order for a plane to take off, it needs lift. Once again you proved my point, since there is NO wind, there will be no lift.

    And with your statement that the runway moves opposite the plane, if both move at opposite directions at the same speed, what happens to the plane? It remains at constant velocity (zero!).

    There is no trick here. It's just simple physics.

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